Pretilt angle direction in a liquid crystal cell

ABSTRACT

A method is disclosed for controlling a pretilt angle direction for a liquid crystal cell comprising the steps of first setting the magnitude of pretilt angle and a plurality of pretilt angle directions in an alignment layer. This first step is achieved by irradiating linearly the alignment layer with polarized or unpolarized UV light. One of the plurality of pretilt angle directions is then selected by exposing the alignment layer to UV light a second time.

This application is a Division of U.S. patent application Ser. No.09/457,388, filed Dec. 9, 1999, which is a Continuation patentapplication of Ser. No. 08/672,183, filed Jun. 27, 1996.

BACKGROUND OF THE INVENTION

The present invention relates to a liquid crystal cell, and particularlyto a method for controlling a pretilt angle direction in liquid crystalcell in which photoreaction occur in a polymerized layer by ultravioletirradiation.

Liquid crystals are the liquids consisting of anisotropic molecules. Theaverage direction of long axes of these molecules is referred as thedirector of LC. The director distribution in a LC bulk is determined byits anchoring on the rigid substrates and characterized by the directionof the axes of easy orientation, corresponding to the minimum of thesurface energy of an LC, pretilt angle between the axis of easyorientation and the substrate plane, and tilt angle between the directorof LC and the substrate plane.

In order to obtain the uniform brightness and high contrast ratio of aliquid crystal display, the LC molecules must be appropriately alignedafter being injected between the substrates of the cell. Not only thevalue of the director tilt but the direction of this tilt (i.e.direction of the axis of easy orientation) is important for normaloperation of LC devices constituting double- and multi-domain structure.Such alignment is achieved by providing an alignment layer on thesurface of the substrate. A rubbing process can be used for aligningliquid crystal molecules. In this rubbing process, a polyamide alignmentlayer is first coated on the substrate and the rubbing is performedmechanically, so that microgrooves are formed on the surface of thealignment layer. The liquid crystal molecules are thus uniformly aligneddue to the intermolecular interaction between the polyamide moleculesand the liquid crystal molecules.

In the above described rubbing process, however, defects are formed inthe microgrooves which cause light scattering and random phasedistortion. Also during the rubbing process, dust and electrostaticcharges are produced in the alignment layer, so that the substrate isdamaged and yield is decreased.

To solve the aforementioned problem, photo-alignment process has beenrecently introduced. As an example of the photo-alignment method, amethod has proposed by KOBAYASHI, etc. (SID 95 DIGEST, p. 877) in whichthe pretilt angle direction is determined by irradiating the UV lighttwice into an alignment layer consisting of polyvinylcinnamate (PVCN)based polymer, as shown in FIGS. 1A and 1B.

In particular, as shown in FIG. 1A, when the linearly polarized UV lightirradiates to alignment layer 15 in the direction perpendicular to thesurface of the substrate 16, the alignment layer 15 becomes aphoto-polymerized due to cross linking between polymer molecules. Thebonding direction of the photo-polymer molecules depends on thepolarization direction of the linearly polarized UV light. The liquidcrystal is thus aligned according to the bonding direction of thephoto-polymer molecules.

Then, the linearly polarized UV light whose polarization direction isperpendicular to the polarization direction of the first UV light isirradiated at an angle φ to the alignment layer 15. The pretilt angle ofthe alignment layer 15 is formed in this step and the magnitude of thepretilt angle varies according to the irradiation angle of the UV light.For example, the pretilt angles are approximately 0.15°, 0.26°, or0.30°, when the irradiation angles are 30°, 40°, or 60°, respectively.

In KOBAYASHI, however, the method has some drawbacks the thermostabilityof the tilt angle on the PVCN based materials is poor, the scope of thepretilt angle is small and does not cover the range needed for anapplications, only polarized exciting light could be used. Moreover, themethod requires rather complicated geometry of the irradiation andsuitable for the only materials revealing the light-induced easy axisdirection perpendicular to the polarization of the exciting light.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method forcontrolling pretilt angle direction for liquid crystal cell in whichcontrol of the pretilt angle direction is easy and large pretilt anglecan be obtained.

In order to achieve the object, the method for controlling pretilt anglefor liquid crystal cell comprises the first step of irradiating UV lightto an alignment layer to form pretilt angle and orient the alignmentaxis and second step of irradiating UV light to the alignment layer inthe direction of oblique to the surface. The order of these step isreversible.

The first and second exposures to UV light can be performed at an angle,preferably 0-60°, or perpendicular to the substrate surface. Moreover,both polarized and unpolarized UV light can be used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are views illustrating the conventional method forcontrolling pretilt angle for liquid crystal cell;

FIG. 2 is a view illustrating an UV light irradiation and birefringencemeasurement apparatus;

FIGS. 3A and 3B are views illustrating method for controlling pretiltangle direction for liquid crystal cell according to first embodiment ofthe present invention;

FIG. 4 is a graph showing pretilt angle dependent on a absorption energyof the UV light;

FIGS. 5A and 5B are views illustrating the method for controllingpretilt angle direction for liquid crystal cell according to secondembodiment of the present invention;

FIGS. 6A and 6B are views showing the method for controlling pretiltangle direction according to third embodiment of the present invention;

FIGS. 7A and 7B are views showing the method for controlling pretiltangle direction according to fourth embodiment of the present invention;

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 is a view illustrating ultraviolet (UV) irradiating andbirefringence measuring apparatus for photo-alignment process. In thisdevice, UV light generated from Hg lamp 11 is linearly polarized througha lens 12 a and a polarizer 17 c, and is directed to the alignment layer15 coated on substrate 16. A laser beam generated from a laser 18 ispulsed by beam by a chopped 23, and then it is polarized by a polarized17 a. The pulsed beam is then transmitted to the alignment layer 15through a compensator 25, through a polarizer 17 b and a lens 12 b, andfinally inputted to a digital oscilloscope 20 so that the birefringencecaused by anisotropy of the alignment layer 15 can be measured.

FIGS. 3A and 3B illustrate first embodiment of the present invention. Inthis embodiment, the alignment layer includes polysiloxane basedmaterial or polyvinylfluorocinnamate (PVCN-F). In this alignment layer,pretilt angle and the direction of the easy axis are determined by onetime irradiation of the UV light. The structural formulas of thepolysiloxanecinnamate and PVCN-F are indicated below. Further, thesematerials can be used in other embodiments of the present invention.

examples of polysiloxanecinnamate include:

Z=OH, CH₃ or a mixture of OH and CH₃

m=10-100

l=1,3,4,5,6,7,8,9 or 10

K=0,1 or 2

X,X₁,Y=H,F,Cl,CN,CF₃,OCF₃,C_(n)H_(2n+1) or OC_(n)H_(2n+1) (n=1−10)

The polymer solution, the polysiloxanecinnamate or PVCN-F solution, isprepared using 1:1 mixture of 1,2-dichloroetane and chlorobenzene. Aconcentration of the solution is 20 g/l. A droplet of this solution isdropped in the center of the substrate and then coated on the whole areaof the substrate by a spin-coating for 20 second at 2,000 rpm. As aresult, a polymer film is deposited on the substrate. The thickness ofthe film, measured by Linnik interferometer, is 1,000 Å, and can becontrolled by changing the concentration of the polymer solution and/orrevolution speed of a spin-coating machine used for spin-coating.

As shown in FIG. 3A, when the linearly polarized UV light 190 isirradiated to the alignment layer 15 in the perpendicular direction ofthe surface of the substrate, the direction of alignment-axis isdetermined to be perpendicular to the polarization direction of the UVlight. Further, two symmetric and bidirectional pretilt angles φ₁ arecreated on both sides of the alignment-axis. The size of the pretiltangle φ₁ is variable and depends on the duration of UV exposure, i.e.,the amount of UV energy absorbed by alignment layer 15.

FIG. 4 is a graph showing the relation between the pretilt angle and theirradiation time of the UV light, i.e., the amount of absorption energyof the UV light. As shown in the figure, the larger the absorptionenergy is, the smaller the pretilt angle becomes. Accordingly, thedirection of the alignment axis is determined by the polarizationdirection of the UV light irradiation, and the size of pretilt angle isdependent upon the amount of UV absorption energy.

After irradiating linearly polarized UV light, another light isirradiated to the layer to the direction oblique to the surface of thealignment layer, that is, one direction of two symmetric pretilt anglesis then selected by irradiating alignment layer 15 at an angle θ₁, 0°<θ₁≦60° with non-linearly polarized UV light, specially unpolarized UVlight 200, such that all the molecules of alignment layer 15 areoriented at one of the symmetric pretilt angle directions, as in FIG.3B. The alignment layer 15 preferably includes polysiloxane basedmaterial or PVCN-F. In this case, the irradiation direction of thesecond UV light forms acute angle with one of two symmetric pretiltangle directions.

FIGS. 5A and 5B illustrate a second embodiment of the present invention.In the figure, unpolarized UV light(wavelength λ=365 nm) 210 irradiatesalignment layer 15 coated on the substrate 16 at an angle θ₂ to thenormal direction of the surface of the substrate 16. Althoughunpolarized UV light is used in this embodiment, the non-linearlypolarized UV light, which means that the light is circularly orelliptically polarized, or the light does not polarized can be usedeither. θ₂ is preferably in the range of 0°<θ₂≦60°. After the initialirradiation shown in FIG. 5A, the molecules of alignment layer 15 areset or oriented with a large number of one sided pretilt angledirections, while the magnitude of the pretilt angles formed by each ofthese directions with the surface is substantially the same. After thesecond irradiation shown in FIG. 5B, however, only one of the pretiltangle directions will be selected.

Thereafter, when linearly polarized UV light 220 irradiates thealignment layer 15 at a perpendicular angle, as shown in FIG. 5B, onlythe pretilt angle direction perpendicular to the polarization directionof this UV light is selected, so that the desired pretilt angledirection can be obtained. Furthermore, the size or magnitude of thepretilt angle produced in the alignment layer 15 varies depending uponthe amount of UV energy absorbed, as noted above with respect to FIG. 4.

FIGS. 6A and 6B illustrate a third embodiment of the present invention.As shown in FIG. 6A, unpolarized UV light 230 irradiates alignment layer15 at an angle θ₃ to the normal direction of the surface of thesubstrate, where the θ₃ is in the range of 0°<θ₃≦60°. Thereafter,linearly polarized UV light 240 irradiates alignment layer 15 again atan angle θ₄, as shown in FIG. 6B, where the θ₄ is in the range of0°<θ₄≦60°.

The resulting orientations of the molecules of alignment layer 15 afterthe steps shown in FIGS. 6A and 6B is similar to that of FIGS. 5A and5B, respectively. Namely, after the irradiation shown in FIG. 6A, themolecules of the alignment layer 15 are oriented at a large number ofone sided pretilt angle directions, as in FIG. 5A. Moreover, as in FIG.5B, only one of these directions is selected after the secondirradiation shown in FIG. 5B. The irradiation direction of the second UVlight forms acute angle with the pretilt angle directions by firstirradiation. In this embodiment, non-linearly polarized UV light,including circularly polarized, elliptically polarized, and unpolarizedUV light, can be used instead of unpolarized UV light. By usingnon-linearly polarized UV light, the method becomes simplified comparedwith the conventional method using linearly polarized UV light.

A fourth embodiment of the present invention is shown in FIGS. 7A and7B. As shown in FIG. 7A, linearly polarized light 250 irradiatesalignment layer 15 at an angle θ₅, 0°<θ₅≦60°, relative to the normal ofthe surfaces of alignment layer 15 to orient the molecules of alignmentlayer 15 at first and second pretilt angle directions, similar to thatshown in FIG. 3A. One of these directions is then selected byirradiating alignment layer 15 at an angle θ₆, 0°<θ₅≦60°, withnon-linearly polarized UV light, specially unpolarized UV light 260,such that all the molecules of alignment layer 15 are oriented at one ofthe first and second pretilt angle directions, as in FIG. 7B. In thisembodiment, the irradiation direction of the second UV light forms acuteangle with one of the first and second pretilt angle directions.

In aforementioned methods according to the present invention, the sizeof the pretilt angle and the two symmetric pretilt angle directions aredetermined by an initial exposing an alignment layer including apolysiloxane based material or PVCN-F to UV light. One of these pretiltangles is then selected by a second exposure to UV light. Accordingly,the pretilt angle can be controlled easily.

While the invention has been described in its preferred embodiments,this should not be construed as limitation on the scope of the presentinvention. Accordingly, the scope of the present invention should bedetermined not by the embodiment illustrated, but by the appended claimsand their legal equivalents.

What is claimed is:
 1. A method for controlling a pretilt angledirection for an alignment layer of a liquid crystal cell, comprisingthe steps of: irradiating said alignment layer on a substrate surface afirst time with linearly polarized ultraviolet light to orient moleculesof said alignment layer in first and second pretilt angle directions,said first and second pretilt angle directions forming first and secondpretilt angles, respectively, relative to a direction parallel to saidsubstrate surface; and irradiating said alignment layer a second timewith non-linearly polarized ultraviolet light to select one direction ofsaid first and second pretilt angle directions.
 2. A method forcontrolling a pretilt angle direction for an alignment layer of a liquidcrystal cell according to claim 1, wherein a magnitude of said first andsecond pretilt angles are determined by a duration of said irradiationstep first time.
 3. A method for controlling a pretilt angle directionfor an alignment layer of a liquid crystal cell according to claim 1,wherein said first and second pretilt angles are of equal magnitudesubstantially and of opposite sign relative to said parallel directionto said substrate surface.
 4. A method for controlling a pretilt angledirection for an alignment layer of a liquid crystal cell according toclaim 1, wherein said alignment layer includes a polysiloxane basedmaterial.
 5. A method for controlling a pretilt angle direction for analignment layer of a liquid crystal cell according to claim 1, whereinsaid alignment layer includes polyvinylfluorocinnamate(PVCN-F).
 6. Amethod for controlling a pretilt angle direction for an alignment layerof a liquid crystal cell according to claim 1, wherein said step ofirradiating said alignment layer a first time include a step ofirradiating said alignment layer to ultraviolet light at an anglesubstantially equal to ninety degree relative to said substrate surface,and said step of irradiating said alignment layer a second time includesa step of irradiating said alignment layer at an angle relative to anormal direction to said substrate surface.
 7. A method for controllinga pretilt angle direction for an alignment layer of a liquid crystalcell according to claim 6, wherein a incident direction of said secondnon-linearly polarized ultraviolet light forms an acute angle with saidone of said first and second pretilt angle directions.
 8. A method forcontrolling a pretilt angle direction for an alignment layer of a liquidcrystal cell according to claim 6, wherein said angle is in the range of0 degree to 60 degree.
 9. A method for controlling a pretilt angledirection for an alignment layer of a liquid crystal cell according toclaim 1, wherein said step of irradiating said alignment layer a firsttime include a step of irradiating said alignment layer to ultravioletlight at a first angle relative to a normal direction to said substratesurface, and said step of irradiating said alignment layer a second timeincludes a step of irradiating said alignment layer at a second anglerelative to a normal direction to said substrate surface.
 10. A methodfor controlling a pretilt angle direction for an alignment layer of aliquid crystal cell according to claim 9, wherein a incident directionof said second non-linearly polarized ultraviolet light forms an acuteangle with said one of said first and second pretilt angle directions.11. A method for controlling a pretilt angle direction for an alignmentlayer of a liquid crystal cell according to claim 9, wherein said firstand second angles are in the range of 0 degree to 60 degree,respectively, relative to a normal direction to said substrate surface.